This application claims the priority of German Application No. 100 59 117.5 filed Nov. 28, 2000, which is incorporated herein by reference.
The invention relates to a deflecting and guiding device for slivers of cotton, chemical fibers or the like and is positioned between first and second spaced roll pairs which clamp the running sliver and which draft the same by virtue of a greater rotating speed of the second roll pair situated downstream of the first roll pair. The device includes a pressure bar positioned between the two roll pairs. The pressure bar deflects the sliver as it runs from the first roll pair to the second roll pair. The two roll pairs together define the principal drafting field.
The invention particularly concerns an optimized configuration of a pressure bar arrangement for improving the guidance of textile fibers in drafting units which are designed for drafting a plurality of simultaneously running slivers and which have a decisive effect on the uniformity of the drafted slivers.
Slivers are drafted by passing them through roll pairs (usually three in number) whose circumferential speed increases in the advancing direction of the sliver. The purpose of doubling the sliver is to even the fineness fluctuations along small and large lengths. In the drafting process those fibers are critical whose length is shorter than the clamping distance between two consecutive roll pairs. Such short fibers are, for a brief period, not clamped either by the leading roll pair nor by the upstream-adjoining the trailing roll pair and may thus move in an uncontrolled manner. These xe2x80x9cfloatingxe2x80x9d fibers are responsible for the drafting waves which negatively affect the uniformity of the slivers. The number of floating fibers is dependent from the fiber length distribution of the material and from the set distance between the clamping nips of two adjoining roll pairs. The greater these distances, the higher the proportion of floating and uncontrolled fibers. For this reason, current drafting units are provided with a stationary pressure bar disposed in the principal drafting field. By means of such an arrangement the floating fibers are better guided and thus a significantly improved sliver uniformity is achieved.
In a device disclosed in German Offenlegungsschrift (application published without examination) 42 42 722, a reduction of deposits on the pressure bar is achieved by an optimization of the pressure bar geometry. The pressure bar which is stationarily supported along the width of the drafting field, has a rectangular cross section whose narrow end faces are semi-circularly rounded.
German Offenlegungsschrift 30 16 409 describes a fiber guiding device which consists of a pressure bar having a bent shoe oriented towards the outlet roll pair. An opening of a suction channel is arranged at the end of the shoe.
xe2x80x9cMelliand Textilberichtexe2x80x9d (xe2x80x9cMelliand Textile Reportsxe2x80x9d) 79 (1998), describes on pages 403, 404, 406 and 407 that the behavior of the fiber motion in the drafting fields is significantly dependent from the delivery speed. At low speeds, as a rule, a uniform fiber motion occurs. At higher speeds, in the middle of the drafting field, a sudden acceleration takes place which is coupled with a significant deterioration of the sliver uniformity. It has been observed that the fiber speed is less constant both along the width of the sliver and over time. In addition, in a part of the slivers a more pronounced alternation of acceleration and deceleration occurs, that is, the fiber acceleration is not continuous. These effects have their cause in an increase in the fluctuation of sliding properties of the material and the fluctuation of the sliver thickness and thus in the uncontrolled guidance of the slivers in the principal drafting field. These fluctuations of the fiber motions are the reason why the slivers become non-uniform.
It is an object of the invention to provide a device of the above-outlined type from which the discussed disadvantages are eliminated. It is a particular object of the invention to optimize the design of known pressure bars for ensuring a better guidance of the slivers, particularly at very high production speeds (greater than 600 m/min).
These objects and others to become apparent as the specification progresses, are accomplished by the invention, according to which, briefly stated, the fiber processing machine includes a sliver drafting unit which has first and second adjoining roll assemblies each defining a nip through which a sliver runs in a sliver advancing direction. The second roll assembly is spaced downstream of the first roll assembly. The nips of the first and second roll assemblies together define a principal drafting field and the drafting direction thereof. A sliver guiding device composed of at least two pressure bars disposed in the principal drafting field for contacting and deflecting the sliver as the sliver advances from the first roll assembly to the second roll assembly.
The pressure bar arrangement according to the invention results in a significantly better guidance of the slivers in the principal drafting field. Further, a reduction of fiber fly of unguided or floating fibers which become loose from the sliver is achieved. In this manner, a significantly reduced number of good fibers will be wasted.
Expediently, the pressure bar according to the invention has, as compared to conventional arrangements, a new geometry which deflects the fibers in the principal drafting field and contacts the fibers over a surface area. The size of the surface area may be selected as a function of the fiber material (material type, fiber length distribution, proportion of floating fibers, etc.). This variant is particularly adapted for slivers having a high proportion of floating fibers and in case the nip distance between adjoining drafting roll pairs is large. The sliver-contacting surface area may be oriented horizontally or may have an inclination to the drafting direction. Expediently, two fiber guiding components having flat sliver-contacting surface areas are used between which the fiber material passes.